The present invention relates to an electric resistance type melting furnace for melting and disposing of incineration ashes which are generated when general material waste and industrial waste are incinerated.
When general material waste and industrial waste are incinerated at a temperature lower than 900.degree. C., incineration ash is generated. The incineration ash contains such substances and metal components, which are detrimental to environment and harmful to humans and animals. The problems related to these harmful substances, such as PCB, dioxin, lead, cadmium, etc., cannot be solved by an incineration furnace using normal type of fuel. If the incineration ash is discharged to reclaimed land, water pollution or soil pollution naturally occurs. In recent years, it has been reported that metal components are eluted due to acidic rain having a pH value of 3, and this adversely affects drinking water or the like.
To solve this problem, various methods have been proposed in the past in order to make the harmful substances harmless by melting the incineration ash. A furnace using oil and gas cannot be used for this purpose because such furnace requires a large quantity of air for combustion and it is necessary to minimize the quantity of oxygen in the furnace. Thus, an electric furnace is used for this purpose. A high frequency furnace belongs to the category of the electric furnace, but it is difficult to create a space of large capacity high frequency furnace. An electric arc furnace is used generates an arc between molten metal and a graphite electrode and produces a melt by the heat thus generated. Due to a high temperature of more than 3000.degree. C., the furnace is often damaged and the electrodes are worn out, this also requires high power consumption. The most desirable furnace for this purpose must be designed in such manner that the furnace is substantially sealed to exclude the intrusion of the air, and it is essential to use an electric resistance-type melting furnace, which generates heat by utilizing electric resistance to inorganic melt itself and to maintain intra-furnace temperature.
A basic design for the electric resistance type melting furnace is described in Japanese Patent No. 1334791 (JP-B-60-56963). A description will be given now of this patented design referring to FIG. 10. On a ceiling unit 102 of a melting furnace 101, three graphite electrodes 103 for 3-phase AC are penetrating the ceiling member 102 in such manner that these electrodes can be moved up and down. Further, on the ceiling unit 102, there are provided a chute 105 for supplying incineration ash 104, an air inlet 106 for supplying combustion air, and an exhaust port 107. A regulator valve 108 for regulating air quantity is provided at the air inlet 106. At the bottom of the melting furnace 101, a tapping hole 110 for discharging metal melt 109 is arranged.
Electric current is supplied to the electrodes 103, which are immersed into the metal melt 109. By Joule's heat generated by using the metal melt 109 as a resistant substance, heat higher than the melting temperature of the metal melt 109, i.e. 1400.degree. C. to 1500.degree. C., is generated. The incineration ash 104 is supplied through the chute 105 to the entire surface of the metal melt 109. The incineration ash 104, thus supplied, is sequentially melted from the portion in contact with the surface of the metal melt 109, and a small quantity of exhaust gas generated at the melting is discharged through the exhaust port 107. When the incineration ash 104 is melted, metal components contained in the incineration ash are accumulated at the bottom as a molten metal layer. Inorganic melt from the incineration ash is separated and forms a glass layer above the molten metal layer. The molten metal and the inorganic melt are sequentially discharged through the tapping hole 110.
In the meantime, in the initial stage of the melting process, it is necessary to tightly fill iron and copper scraps into the electric resistance type melting furnace around the electrodes 103 at the bottom of the melting furnace 101 and to supply electric current to the graphite electrodes 103 to form the metal melt 109. However, the conventional type electric resistance-type melting furnace, as described above, is low in strength because the electrodes 103 are made of graphite, and when iron or copper scraps are tightly filled, graphite electrodes are destroyed. Also, graphite electrodes may be destroyed when electric current is continuously supplied under high temperature conditions.
Also, in the conventional electric resistance-type melting furnace, as described above, the distance between the electrodes 103 is fixed, and melting temperature of the metal melt 109 cannot be accurately controlled. This leads to higher power consumption and an inability to accurately determine the timing of when to charge incineration ash and also results in longer processing time.
Further, in the conventional electric resistance-type melting furnace as described above, the electrodes 103 are designed with a cylindrical shape regardless of the material used, and a 3-phase electrode is adopted. This means that three cylinder-type electrodes are used at all times. Therefore, electric current between the electrodes is distributed in form of a triangle. Electric current is concentrated at the portion where the distance between the surfaces of the cylinder-type electrodes is shortest. In order to melt the material within a short time, the electrodes must be brought closer to each other. As a result, the melting volume of the metal melt 109 is reduced. Further, unless electric current between the electrodes is distributed evenly, the power factor is decreased, and it is necessary to have a reactor or a condenser for power factor improvement.